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Abstract

Hydrangea macrophylla cultivars widely differ in their relative abundance and duration of flower production. However, the reasons for this variation are not well understood. This study consisted of 3 experiments to determine where and when floral induction occurred on commercial cultivars and in the newer re-flowering hydrangeas. It also determined the patterns of floral development of the induced buds and the effect of different pruning times on growth and development of the plants. In evaluations of dormant shoots in 18 cultivars, flower development was very consistent in terminal buds, and occurred in 100% of the terminal buds for all of the cultivars with the exception of ‘Ayesha’ (33%). In contrast, lateral buds showed a wide variation in flower development among different genotypes; and the percentage of induction range from 0 to 100%. Flower development was more advanced in terminal than in lateral buds. In the second experiment examining shoots throughout an annual cycle, cultivars had floral primordia initiated in lateral buds at the first sampling period prior to receiving cold or short days. The degree of induction and development varied according to the cultivar and evaluation (harvest) period. Although, differences were found among some cultivars and evaluation periods, most of the cultivars reached their maximum flower potential and floral development by the time they reached the dormant period (leaf abscission), with no further induction or development during the quiescent period. The results suggested that some cultivars have minimal or no photoperiodic/temperature requirements to induce flowering. These studies indicated that genotypic variation in terminal and lateral floral induction, differences in floral development, and low or minimal inductive conditions (e.g. temperature, photoperiod) required for some cultivars may explain the ability of some cultivars to have a greater abundance and duration of flower production. The information provided for this study could aid the industry and gardeners in developing cultural practices (chemical treatments or pruning practices) to promote lateral bud-break throughout the growing season, thereby enhancing the production of flowers and extending the blooming season of some cultivars.

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